A complex programmable logic device is an innovative product compared to earlier logic devices like programmable logic arrays (PLAs) and Programmable Array Logic (PAL). The earlier logic devices were not programmable, so the logic was built by combining multiple logic chips together. A CPLD has a complexity between PALs and field-programmable gate arrays (FPGAs).
CPLD vs FPGA comparison summary
| No. | CPLD | FPGA |
|---|---|---|
| 1 | Instant-on. CPLDs start working as soon as they are powered up | Since FPGA has to load configuration data from external ROM and setup the fabric before it can start functioning, there is a time delay between power ON and FPGA starts working. The time delay can be as large as several tens of milliseconds. |
| 2 | Non-volatile. CPLDs remain programmed, and retain their circuit after powering down. FPGAs go blank as soon as powered-off. | FPGAs uses SRAM based configuration storage. The contents of the memory is lost as soon as power is disconnected. |
| 3 | Deterministic Timing Analysis. Since CPLDs are comparatively simpler to FPGAs, and the number of interconnects are less, the timing analysis can be done much more easily. | Size and complexity of FPGA logic can be humongous compared to CPLDs. This opens up the possibility less deterministic signal routing and thus causing complicated timing scenarios. Thankfully implementation tools provided by FPGA vendors have mechanisms to assist achieving deterministic timing. But additional steps by the user is usually necessary to achieve this. |
| 4 | Lower idle power consumption. Newer CPLDs such as CoolRunner-II use around 50 uA in idle conditions. | Relatively higher idle power consumption. |
| 5 | Might be cheaper for implementing simpler circuits | FPGAs are much more capable compared to CPLDs but can be more expensive as well. |
| 6 | More "secure" due to design storage within built in non-volatile memory. | FPGAs that use external memory can expose the IP externally. Many FPGA vendors offer mechanisms such as encryption to combat this. Design specific protection mechanisms also can be implemented. |
| 7 | Very small amount of logic resources. | Massive amount logic and storage elements, with which incredibly complex circuits can be designed. FPGAs have thousands times more resources! This point alone makes FPGAs more popular than CPLDs. |
| 8 | No on-die hard IPs available to offload processing from the logic fabric. | Variety of on-die dedicated hardware such as Block RAM, DSP blocks, PLL, DCMs, Memory Controllers, Multi-Gigabit Transceivers etc give immense flexibility. This is not even thinkable with CPLDs. |
| 9 | Power down and reprogramming is always required in order to modify design functionality. | FPGAs can change their circuit even while running! (Since it is just a matter of updating LUTs with different content) This is called Partial Reconfiguration, and is very useful when FPGAs need to keep running a design and at the same time update the it with different design as per requirement. This feature is widely used in Accelerated Computing. |
Where to use CPLD or FPGA
When a design requires simple glue-logic or similar functionality which doesn’t need to be changed much, or when you need an instant-on circuit, then go for CPLDs. Otherwise, for most other applications FPGAs are generally preferred. Sometimes you can find both CPLD + FPGA in a design. In those designs, CPLDs generally do simple glue-logic as mentioned before, and are responsible for “booting” the FPGA as well as controlling reset and boot sequence of complete board.
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